On average our window cleaners work a 35 hour week, Mon - Thurs (8-4) 7.5 hrs(domestic and commercial), Fridays (7-12) 5 hrs (weekly commercial). There are always exceptions to this, sometimes commercial work needs to be done at weekends.
However if possible I very rarely have them work at weekends, though we did in Jan to catch up after the snow and holidays over Christmas.
In general I agree with Squeaky, weekdays for work, weekend for home life (where possible)
If you are getting to the point where your window cleaning is causing you to work at weekends, or you are taking 5-6 weeks to do a 4 week rota, then you need to look at taking someone on to help out.